A field effect transistor is a device which is widely used as a unit electronic element of a semiconductor memory integrated circuit, a high-frequency signal amplification element, a liquid crystal driving element or the like. It is an electronic device which is most practically used in recent years.
Of these, in particular, with a remarkable progress in displays in recent years, a thin film transistor (TFT) is widely used as a switching device which applies a voltage to a display element to drive a display for liquid crystal displays (LCD), electroluminescence displays (EL) and field emission displays (FED).
As the material of the above-mentioned thin film transistor, a silicon-based semiconductor is widely used. In general, crystalline silicon is used in a high-frequency amplification element, an integrated circuit element or the like, which require high-speed operation. In a liquid crystal driving element or the like, amorphous silicon is used to meet the requirement for an increase in area.
However, crystalline silicon is required to be heated at a high temperature, for example, 800° C. or higher, or by means of an excimer laser for crystallization. Therefore, it is difficult to apply it to a large-area substrate. In addition, there is a problem that a large amount of energy and a large number of steps are required in production. Further, since crystalline silicon is normally restricted to a TFT with a top-gate configuration, a reduction in production cost such as a decrease in number of masks is difficult.
On the other hand, an amorphous silicon semiconductor (amorphous silicon) which can be formed at a relatively low temperature has a small mobility of about 0.5 cm2/Vs, and has a low switching speed as compared with a crystalline silicon-based thin film. Therefore, a problem may arise that a large-area, highly precise and high-frequency animation cannot be displayed. Further, there is a problem that a field effect transistor using amorphous silicon has low stability (reliability) to direct current stress and hence it is difficult to apply it to a self-emission type display element such as an organic EL device which is driven at direct current.
Today, as a switching element for driving a display, a device using a silicon-based semiconductor film constitutes the mainstream due to various excellent performances including improved stability and processability of a silicon thin film and a high switching speed. Such a silicon-based thin film is generally produced by the chemical vapor deposition (CVD) method.
Conventional thin film transistors (TFT) have an inverted-staggered structure in which, on a substrate formed of glass or the like, a gate electrode, a gate-insulating layer, a semiconductor layer such as a hydrogenated amorphous silicon (a-Si:H) film, a source electrode and a drain electrode are sequentially stacked. This inverted-staggered type TFT is used, in a field of large-area devices including an image sensor, as a driving element for flat panel displays represented by active matrix-type liquid crystal displays. However, in these applications, with an improvement in function (corresponding to a large-area, high precise and high-frequency display), a further increase in operation speed is demanded even for thin film transistors.
Under such circumstances, an oxide semiconductor using an oxide has attracted attention as a semiconductor with which improvement in transistor performance (mobility and stability) and an increase in area can be attained simultaneously.
However, of such metal oxide semiconductors, a conventional metal oxide semiconductor using zinc oxide has poor TFT characteristics such as a low mobility, a small on-off ratio, a large amount of current leakage, unclear pinch-off and tendency of becoming normally-on. In addition, due to poor chemicals resistance, metal oxide semiconductors have problems that they are hard to be subjected to wet etching or the like, and hence, the production process or the use environment was restricted.
Furthermore, since it is required to form an oxide semiconductor into a film at a high pressure in order to improve performance, the film-forming speed is slow and a high temperature treatment at 700° C. or higher is required. Moreover, in the case of a semiconductor with a top-gate configuration, many restrictions are imposed for practical use since the film thickness of the oxide semiconductor is required to be 50 nm or more.
In order to solve such a problem, a field effect transistor using an amorphous oxide semiconductor which is composed of indium oxide and zinc oxide or an amorphous oxide semiconductor which is composed of indium oxide, zinc oxide and gallium oxide has been studied. However, if gallium (Ga) is not added, stability to environments such as moisture resistance is lowered, and when the added amount of Ga increased, there is a possibility that the TFT characteristics, such as mobility and S value, may be deteriorated. Moreover, since Ga is a costly rare metal, stable supply thereof is difficult.
Under such circumstances, as an oxide semiconductor which does not contain Ga, a field effect transistor using an amorphous oxide semiconductor which is composed of indium oxide, zinc oxide and tin oxide has been studied (for example, see Patent Document 1).
Although a field effect transistor using tin oxide has been studied for many years, it has not been put in practical use due to a high off current and a low mobility. The reason therefor is considered to be that, in tin oxide, a lower oxide (SnO or the like), which is an insulator, tends to be generated easily. For these reasons, it is considered that tin oxide is not suitable as a semiconductor material. Actually, in a field effect transistor using an amorphous oxide semiconductor which is composed of indium oxide, zinc oxide and tin oxide containing tin as a main component, the off current and the hysteresis are large and the threshold voltage (Vth) is significantly negative. Furthermore, although the mobility can be improved by a heat treatment, since the threshold voltage tends to shift in the negative direction greatly according to a heat treatment temperature, there were problems which inhibit practical use such as a large variation in properties of each transistor, poor reliability or the like (for example, see Non-Patent Document 1).
Moreover, an amorphous oxide semiconductor composed of indium oxide, zinc oxide and tin oxide which is obtained by co-sputtering without using tin as a main component has been studied. With this semiconductor, if zinc is contained in an amount of 25 atom % or more, the mobility is lowered to cause the threshold voltage to increase, and if zinc is contained in an amount of less than 25 atom %, the S value is increased to cause the threshold voltage to be negative. For these reasons, it is believed that it is difficult to find out a composition ratio which can produce a field effect transistor with excellent transistor characteristics (for example, see Non-Patent Document 2).
Under such circumstances, it is believed that, with an amorphous oxide semiconductor which is composed of indium oxide, zinc oxide and tin oxide, production of a field effect transistor suitable for practical use in a display panel or the like is believed to be difficult.